Dry and wet deposition rates

The oceanic burden in December 2004 shows the contamination of the ocean after 50 years of PFOA emissions (Figure 3.14). Highest PFOA burden is located in the northern Atlantic, Mediterranean, and the Arctic ocean. Contaminations of the Atlantic, Mediterranean and Pacific can be related to the vicinity to the oceanic source. PFOA in remote regions, however, such as in the Arctic must have been transported via atmosphere or ocean. MPI-MTCM does not simulate degradation of PFOA from volatile, highly mobile precursor substances, that contribute to the ocean burden in the Arctic by deposition. Then annual dry and wet deposition rates of PFOA in the model are small compared to the mass emitted directly to the ocean. This implies that the burden in the Arctic is results mainly from oceanic long-range transport. 

Figure 3. A comparison between dry and wet deposition rates of sulfur, as computed from the trial dry deposition data reported here and from records of MAP3S precipitation chemistry network, for Oak Ridge, Tennessee. Data are reported as average weekly values, computed for each month.

According to the measurements of Rancher and Kritz (1980), the total gas phase bromine concentrations are comparable to the particulate concentrations. In contrast, Moyers and Duce (1972) found 4-10 times higher concentrations of gas phase than particulate bromine. One explanation for the discrepancies might be the differences in dry and wet deposition rates. Rancher and Kritz (1980) could distinguish a diurnal cycle in particulate Br with nighttime values that were about twice as high than daytime values. 

Impurities travel from atmosphere to ice sheet surface either attached to snowflakes or as independent aerosols. These two modes are called wet and dry deposition, respectively. The simplest plausible model for impurity deposition describes the net flux of impurity to ice sheet (which is directly calculated from ice cores as the product of impurity concentration in the ice, Ci, and accumulation rate, a) as the sum of dry and wet deposition fluxes which are both linear functions of atmospheric impurity concentration Ca (Legrand, 1987) ... 

The Comprehensive Acid Deposition Model (CADM) has been created for calculation of dry and wet deposition of sulfur species over South Korea (Park et al 1997,1999a). This model presents quantitative assessment of the acidity loading and alterations in deposition rates. 

The Warren Spring Laboratory believe that it is not possible to measure directly the dry deposition of acidity but that to determine the extent of dry deposition one must measure the species responsible for acidity. Therefore in order to assess the annual dry deposition of SO2/ an average deposition of 8 mm S was used so that 20 pg/m SO2 gives a deposition rate of 2.5 g/ m /year. Such calculations indicate that the contribution of dry and wet deposition to total sulphur depositions are approximately equal over significant areas of the UK and that dry deposition is relatively more important in central and eastern England, while wet deposition predominates in the north and west of Scotland. 

There are chemical reactions between the released contaminant and ambient air or surfaces. If the released contaminant reacts, any reacted material can no longer be considered airborne (although the reaction products may also be hazardous), and so chemical reactions effectively reduce the rate or amount of airborne contaminant. Some reactions can be characterized as dry or wet deposition. 

Airborne salinity can be determined using different methods. In corrosion research the standard method (Wet Candle method) is established in ISO-9225 1992 [33] however, it is not the only method traditionally used. In the case of Cuba it has been widely used the method named as dry plate method, consisting in the employment of a dry cotton fabric of known area exposed under a shed. The amount of chloride deposition on the gauze is determined analytically at the end of the exposure period (two months) and the deposition rate is calculated. 

Selenium compounds released to the atmosphere can be removed from it by dry or wet deposition to soils or to surface water. The annual wet deposition rate of selenium at two rural/agricultural sites in Queenstown, Maryland and St. Mary s, Maryland were 287 and 140 pg/nr-year. respectively (Scudlark et al. 1994). Selenium concentrations ranging from 0.04 to 1.4 pg/L have been detected in rain and snow (Hashimoto and Winchester 1967). Kubota and coworkers (1975) reported selenium concentrations of 0.02-0.37 pg/L in rainwater at several locations in the United States and Denmark. Selenium was detected at average concentrations of 5.60-7.86 pg/L during four rainfall events in Riyadh, Saudi Arabia (Alabdula aly and Khan 2000). 

Strontium released into the atmosphere from natural and anthropogenetic activities is transported and redeposited on the earth by dry or wet deposition. Dry deposition results from gravitational settling, impact, and sorption on surfaces (NCRP 1984). Experimental data on dry deposition of strontium, present in the ambient atmosphere, is limited. Rain, sleet, snow, or other forms of moisture can wash airborne particles containing strontium from the atmosphere by the process of wet deposition. Wet deposition depends on conditions such as particle solubility, air concentration, rain drop size distribution, and rain fall rate (NCRP 1984). Hirose et al. (1993) examined the mechanism of aerial deposition of 90Sr derived from the Chernobyl accident, and found that 96% of atmospheric 90Sr returned to earth as wet deposition. 

Two other source terms need Co be considered. First, there Is the possibility of dry or wet deposition of a chemical species to the water surface from the atmosphere. The rate of dry deposition depends on Che concentration of Che species In Che gas phase, the concentration In surface water, Che Henry s Law constant, and the reactivity of Che species with seawater constituents, as well as the wind speed and the aerosol In the air adjacent to the water surface. e rate of wet deposition depends on Che concentration of the species In the precipitation and Che frequency and duration of precipitation events. Little Information is currently available about these processes, although there Is some evidence that a rainstorm can Inject significant quantities of 2 2 upper... 

Sulfur oxides and other corrosive species are brought to react with the zinc surface in two ways dry deposition and wet deposition. Sulfur dioxide has been observed to deposit on a dry surface of galvanized steel panels until a monolayer of SO2 formed (Maato, 1982). In either case, the sulfur dioxide that deposits on the surface of the zinc forms sulfurous or other strong acids, which react with the film of zinc oxide, hydroxide, or basic carbonate to form zinc sulfate. The conversion of sulfur dioxide to sulfur-based acids may be catalyzed by nitrogen compounds in the air—usually referred to collectively as NQt compounds—and it is believed that this factor may affect corrosion rates in practice. The acids partially destroy the Film of corrosion products, which will then re-form from the underlying metal, so causing continuous corrosion by an amount equivalent to the film dissolved, hence to the amount of sulfur dioxide absorbed. Above about 85% RH, corrosion rates increase further—probably as a result of the formation of basic zinc sulfates. 

To determine the contribution of dry versus wet deposition, Goodwin initiated laboratory tests using synthetic rainfall of different pH values. Dry deposition was attained by setting panels in a louvered box outdoors. Rain intensities between 0 and 12 mm/h were used. This technique made it possible to vary sulfate concentration independently of pH. Rolled pure zinc panels were used for all tests. Atmospheric corrosion rates of zinc is Scandinavia, when only dry deposition was allowed, were measured to be 12 g/m / year based on an exposure period of 2.5 months. Exposure of these dry panels to artificial rainfalls with pH values of 4.5 and 3.5 decreased their corrosion rates. Only when rainfall pH was lowered to 2.5 did zinc corrosion rate increase over that seen with solely dry deposition (Fig. 2.7). 

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